The present invention relates to a novel speed control system for controllinq a DC motor, and in particular to a motor speed control system having improved stability of speed control against load fluctuation. The motor speed control system according to this invention is particularly applicable in the art of electrically driven sewing machines for controlling revolution oi an upper drive shaft driven by an electromotor.
As having been well known in the art, a remote-controller is detachably connected to an electric or electronic sewing machine and operated by the sewing machine operator to control revolution of a motor driven to rotate an upper drive shaft and therefore to reciprocate a fabric penetrating needle, to thereby control operating speed of the sewing machine.
When the revolution of the sewing machine motor is under control of a prior art control system, even with so-called PI (proportion-integration) or PID (proportion-integration differential) control system, speed of rotation of an actual revolution will be greatly varied due to load fluctuation occuring during rotation of the motor especially at low speeds. More particularly, the actual rate of revolution will be much decreased when a pointed end of the needle just enters a fabric, and the rate of revolution again becomes greater when the needle is pulled apart from the fabric. Such sudden variation of the motor revolution would not only make a harsh and unpleasant noise during sewing machine operation but also make it difficult to stop the sewing machine at a predetermined needle position under precise control. Because the load fluctuation arising during sewing machine operation wi11 be so great, a control system employed to control the sewing machine motor must provide improved characteristics as compared with similar control systems conventional in general electric motors. Undesirable variation of the rate of motor revolution would also be caused by unstabilized voltage values supplied to the motor. Further, the sewing machine motor revolution may be selected as desired by the operator within a considerable range so that the control system for the sewing machine motor should control the motor revolution to follow a change in the selected revolution with high reliability.